Photophysical processes effects

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Effects of intermolecular photophysical processes on fluorescence emission... 

Effects of intermolecular photophysical processes on fluorescence emission Box 4.1 Theories of diffusion-controlled reactions... 

I 4 Effects of intermoiecuiar photophysical processes on fuorescence emission... 

It should be emphasized that best design (each application corresponding to a particular design), proper choice, and correct use of fluorescent probes require a thorough knowledge of the basic phenomena involved in ion recognition medium effect on complexation equilibrium, fundamental photophysical processes, and possible changes from other causes than complexation. 

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

Since most chemical reactions if carried to completion will result in deterioration of polymer properties, it is desirable to be able to ensure that all of the energy is dissipated in photophysical processes and to eliminate the photochemical reactions. However, in some cases, such as in the manufacture of photographic resists, it is desirable to maximize the photochemical effects. The polymer chemist is particularly concerned with the problem of how the relative efficiency of these various processes may be affected by the polymeric nature of the molecules he uses. 

Photophysical processes are commonly encountered and can participate in such important phenomena as energy harvesting (antenna effect), which is the basis for natural photosynthesis and artificial systems mimicking it [4-21] (see below). 

To compete effectively with the photophysical processes, the chemical reactions from the highly energetic CT states should be very fast, otherwise the MC states become populated. On the other hand, redox processes may sometimes occur from other than CT excited states. The phenomenon is a consequence of redox potential changes after excitation, which make the entity in any excited state a much stronger oxidant and a much stronger reducer than the ground state complex, eg the standard oxidation potential of the [Fe(bpy)3]2+ complex is 1.05 V in the ground state and... 

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